1. Field of the Invention
The present invention relates generally to an optical waveguide fiber, and particularly to an optical waveguide fiber that compensates dispersion slope in a telecommunications link.
2. Technical Background
Dispersion compensation techniques in telecommunications systems or links have been used successfully. A technique useful in links already installed is one in which total dispersion (also called chromatic dispersion) is compensated by an appropriately designed waveguide fiber formed into a module that can be inserted into the link at an access point such as an end of the link. The compensating waveguide fiber can be designed to allow operation in, for example, the 1550 nm operating wavelength window of a link originally designed for the 1310 nm operating window.
A disadvantage of compensating with a module is that attenuation and nonlinear penalties are added to the link without increasing the useful link length. Also some of the refractive index profile designs for dispersion compensation are more difficult to manufacture and have higher attenuation than the fibers making up the link.
Another dispersion compensation scheme is to include both positive and negative dispersion fibers in the cables of the link. Each cable can contain both positive and negative total dispersion waveguide fibers, or the link can be formed using cables having only positive dispersion together with cables having only negative dispersion. The relatively high attenuation and low effective area of the negative dispersion fiber can be a problem in this scheme as it is in the dispersion compensating module solution. Also the cable inventory must be managed carefully, because replacing or repairing a cable involves tracking of another variable (the sign of the dispersion of fibers in the cable). In certain profile designs a mismatch of mode fields between the positive and negative total dispersion fibers exists and results in excessive splice or connecting losses.
There is therefore a need for a total dispersion compensating strategy in which the compensating fiber is a part of the link length and the problem of the compensating fiber producing excess link attenuation is addressed. Furthermore, a solution that includes introducing negative dispersion cabled fiber into the link must offer a benefit that offsets the cost of cable inventory management and that does not introduce excess splice loss into the link.
A further desired characteristic of a total dispersion compensation solution is that the compensation be effective over an extended bandwidth to facilitate use of wavelength division multiplexed link architectures.
The following definitions are in accord with common usage in the art.
The refractive index profile is the relationship between refractive index or relative refractive index and waveguide fiber radius.
A segmented core is one that is divided into at least a first and a second waveguide fiber core portion or segment. Each portion or segment is located along a particular radial length, is substantially symmetric about the waveguide fiber centerline, and has an associated refractive index profile.
The radii of the segments of the core are defined in terms of the respective refractive indexes at respective beginning and end points of the segments. The definitions of the radii used herein are set forth in the figures and the discussion thereof.
Total dispersion, sometimes called chromatic dispersion, of a waveguide fiber is the sum of the material dispersion, the waveguide dispersion, and the inter-modal dispersion. In the case of single mode waveguide fibers the inter-modal dispersion is zero.
The sign convention generally applied to the total dispersion is as follows. Total dispersion is said to be positive if shorter wavelength signals travel faster than longer wavelength signals in the waveguide. Conversely, in a negative total dispersion waveguide, signals of longer wavelength travel faster.
The effective area is
Aeff=2xcfx80(∫E2rdr)2/(∫E4rdr),
where the integration limits are 0 to ∞, and E is the electric field associated with light propagated in the waveguide.
The relative refractive index percent, xcex94%=100xc3x97(ni2xe2x88x92nc2)/2ni2, where ni is the maximum refractive index in region i, unless otherwise specified, and nc is the average refractive index of the cladding region. In those cases in which the refractive index of a segment is less than the average refractive index of the cladding region, the relative index percent is negative and is calculated at the point at which the relative index in most negative unless otherwise specified.
The term xcex1-profile refers to a refractive index profile, expressed in terms of xcex94 (b) %, where b is radius, which follows the equation,
xcex94(b)%=xcex94(b0)(1xe2x88x92[¦bxe2x88x92b0¦/(b1xe2x88x92b0)]xcex1),
where b0 is the point at which xcex94(b)% is maximum, b1 is the point at which xcex94(b)% is zero, and b is in the range bixe2x89xa6bxe2x89xa6bf, where delta is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number.
The pin array bend test is used to compare relative resistance of waveguide fibers to bending. To perform this test, attenuation loss is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two attenuation measurements. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. The waveguide fiber is caused to pass on opposite sides of adjacent pins. During testing, the waveguide fiber is placed under a tension just sufficient to make the waveguide conform to a portion of the periphery of the pins. The test pertains to macro-bend resistance of the waveguide fiber.
A waveguide fiber telecommunications link, or simply a link, is made up of a transmitter of light signals, a receiver of light signals, and a length of waveguide fiber having respective ends optically coupled to the transmitter and receiver to propagate light signals therebetween. A link can include additional optical components such as optical amplifiers, optical attenuators, optical switches, optical filters, or multiplexing or demultiplexing devices. One may denote a group of inter-connected links as a telecommunications system.
One aspect of the present invention is a single mode optical waveguide fiber, having a core region and a surrounding clad layer. The reference to single mode waveguide fiber means that the fiber in cable form usually will carry only a single mode over the range of operating wavelengths. Persons skilled in the art understand that single mode operation also includes cases in which more than one mode is propagated but that the higher order modes may are strongly attenuated and so do not travel in the waveguide more than a few kilometers. The waveguide fiber in accord with the invention may also be used in a wavelength range where a few modes are propagated the full link length and the few modes are dispersion compensated. The core region includes at least three segments, a central segment beginning at the centerline of the waveguide fiber, and two annular segments surrounding the central segment. In one embodiment, the profile has four segments, a central segment, surrounded by a first, second and third annular segment. Each of the segments is characterized by a refractive index profile, a relative refractive index, and an inner and an outer radius. The respective segment characteristics are selected to provide a waveguide fiber having a total dispersion at 1550 nm in the range of xe2x88x9230 ps/nmxe2x88x92km to xe2x88x9260 ps/nmxe2x88x92km and preferably in the range of xe2x88x9230 ps/nmxe2x88x92km to xe2x88x9248 ps/nmxe2x88x92km, total dispersion slope at 1550 nm in the range of xe2x88x920.09 ps/nm2xe2x88x92km to xe2x88x920.18 ps/nm2xe2x88x92km and preferably in the range of xe2x88x920.09 ps/nm2xe2x88x92km to xe2x88x920.15 ps/nm2xe2x88x92km, an effective area at 1550 nm greater than 25 xcexcm2, and attenuation at 1550 nm less than or equal to 0.30 dB/km. In a preferred embodiment, the attenuation at 1550 is less than or equal to 0.26 dB/km.
The respective relative indexes, symbolized beginning at the central segment as xcex940, the first annular segment (4 in FIG. 1) xcex941, and second annular segment (6 in FIG. 1) xcex942, are related by the inequalities, xcex940 greater than xcex942 greater than xcex941, and xcex941 less than 0.
In an embodiment of the single mode optical waveguide fiber in accord with the invention, the central segment has relative index percent in the range of 0.8% to 1.4% and preferably in the range 0.9% to 1.2%, the first annular segment has relative index percent in the range of xe2x88x920.3% to xe2x88x920.5% and preferably xe2x88x920.35% to xe2x88x920.45%, and the second annular segment has relative index percent in the range of 0.20% to 0.45%. The respective radii associated with this embodiment are for the central segment an inner radius zero and outer radius, r0, in the range 1.8 xcexcm to 3.0 xcexcm, for the first annular segment an inner radius r0 and outer radius in the range r0+1.5 xcexcm to r0+3.0 xcexcm, and for the second annular segment a center radius in the range 4.5 xcexcm to 10 xcexcm and a width, measured between two points defined by the intersection of the second annular segment refractive index profile with a horizontal line drawn at the half relative index percent value of the second annular segment refractive index profile, in the range of 0.3 xcexcm to 4.0 xcexcm.
In another embodiment in accord with the invention, the central segment of the single mode optical waveguide fiber includes a SiO2 layer at the interface of the central segment and the first annular segment. This SiO2 layer is no thicker than 1.5 xcexcm. The composition of the layer ranges from pure SiO2 to 90% SiO2.
In a further embodiment of the waveguide fiber profile, there is a flattened region of refractive index beginning at the outer radius of the first annular segment. The width of this region is no greater than 5.0 xcexcm.
In yet another embodiment in accord with the invention, the clad layer adjacent the core region has a refractive index less than that of SiO2. This portion of the clad layer has a thickness no greater than 20 xcexcm. For most refractive index profile designs of optical waveguide fibers, no light is present at a radius about 20 xcexcm greater than the core radius.
A second aspect of the invention is a telecommunications link which makes use of two types of waveguide fibers. A first waveguide type has positive total dispersion and positive total dispersion slope. A second type, made in accord with the invention, has negative total dispersion and negative total dispersion slope. Combining the two fiber types in a link allows one to compensate for accumulated positive dispersion in the first waveguide type by using, in the link, an appropriate length of negative total dispersion waveguide fiber. The difference in sign of the respective slopes of the first and second waveguide types provides for total dispersion compensation over an extended range of operating wavelengths. In addition, the negative dispersion waveguide fiber can provide a net negative dispersion in each span which mitigates nonlinear penalties due to modulational instability and four wave mixing. This accumulated negative dispersion is then compensated periodically by single spans of the positive dispersion waveguide fiber.
The link includes a transmitter that provides light signals, a receiver that receives the light signals, and at least two lengths of optical waveguide fiber optically coupled between the transmitter and receiver to transport the light signals. At least one of the waveguide fiber lengths has positive total dispersion and total dispersion slope. At least one of the waveguide fiber lengths has negative total dispersion and negative total dispersion slope. The length, total dispersion, and total dispersion slope of the fibers are chosen to provide a link length having a magnitude (as used herein, magnitude refers to absolute value of either a positive or negative total dispersion or total dispersion slope) of total dispersion and total dispersion slope less than 10 ps/nmxe2x88x92km and 0.01 ps/nm2xe2x88x92km, respectively. The combination of fibers having total dispersion of different sign serve to reduce or eliminate the signal dispersion. Because the fibers also have total dispersion slope of different sign, the canceling of signal dispersion takes place over an extended wavelength range.
In an embodiment of the link, the signal dispersion cancellation is effective over a wavelength range 1280 nm to 1650 nm so that the operating window includes wavelengths near 1310 nm as well as the C-band (1530 nm to 1565 nm) and L-band (1565 nm to 1650 nm). The dispersion data show that operation over this very wide wavelength band is possible.
In another embodiment of the link, the optical waveguide fiber having positive total dispersion and total dispersion slope is longer than the optical waveguide fiber having negative total dispersion and total dispersion slope. A preferred embodiment is one in which the positive total dispersion fiber is at least twice as long as the negative total dispersion fiber. Because of the characteristics of the refractive index profile of the negative total dispersion fiber, this fiber generally exhibits a higher attenuation relative to that of the positive total dispersion fiber. Therefore, the link attenuation is reduced when dispersion compensation can be achieved using a shorter length of negative total dispersion fiber.
In a further embodiment of the invention, the link is constructed so that the negative total dispersion fiber is farthest from the transmitter. The advantage of this construction is due to the higher effective area of the positive total dispersion waveguide compared to the effective area of the negative total dispersion waveguide. Non-linear dispersion effects, such as cross phase modulation and four wave mixing, are known to depend upon the ratio of power density in the waveguide fiber to effective to effective area of the fiber. By placing the higher effective area waveguide fiber nearest the transmitter, the higher power signal propagates in the larger effective area fiber. The signal is attenuated before reaching the lower effective area, negative total dispersion fiber so that the non-linear dispersion effects are kept to a minimum.
In telecommunications links designed for two way signal propagation in the waveguide fiber, the non-linear effects are minimized by placing the lower effective area waveguide fiber in the center of the link between two segments of the link constructed of the higher effective area waveguide fiber.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.